The effect of diet composition on post-absorptive (15 h fast) fatty acid and glucose metabolism was investigated in five healthy men after 2 weeks on a low-carbohydrate (Low-CHO) diet (30 % energy intake from carbohydrates, 55 % from fat, 15 % from protein) and after 2 weeks on a high-carbohydrate (High-CHO) diet (energy intake 75, 10 and 15 % from carbohydrates, fat and protein respectively). The diets were isoenergetic and comprised real foods. Stable-isotope tracer methodology and indirect calorimetry were employed to measure glucose and fatty acid kinetics and oxidation. The relative contribution of carbohydrate to the total energy expenditure was significantly higher after the High-CHO diet. After the High-CHO diet, total and plasma fatty oxidation (2.4 (SE 0.7) and 2.1 (SE 0.4) micromol/kg per min respectively) were significantly lower than after the Low-CHO diet (4.8 (SE 0.5) and 4.6 (SE 0.8) micromol/kg per min for total and plasma fatty oxidation respectively). The rate of appearance (Ra) of non-esterified fatty acids (NEFA) in plasma and the arterial NEFA concentration were both significantly lower following the High-CHO than the Low-CHO diet. However, even after the High-CHO diet, NEFA Ra was threefold higher than plasma fatty acid oxidation. Thus, the decrease in fatty acid oxidation after consumption of a high-carbohydrate diet for 2 weeks in healthy men is unlikely to result from decreased fatty acid delivery to the tissues. Glucose Ra and arterial plasma glucose concentration were similar after the two diets. After the High-CHO diet, arterial lactate concentration was higher and total carbohydrate oxidation rate well exceeded glucose Ra in plasma. Therefore, alterations in intracellular mechanisms may limit fatty acid oxidation after high-carbohydrate diets.